Turbine propeller control system



June 2, 1953 G. P. KNAPP ET AL 2,640,550

' TURBINE PROPELLER CONTROL SYSTEM Filed July 24, 1948 7 Sheets-Sheet 1 TORQUE METER INVENTORS GEORGE P. KNAPP WILUAM EBURNS W\\.\.\AM P. ROBNNS RAYMONDT-ZWACK ATTORNEY.

June 2, 1953 G. P. KNAPP ET AL TURBINE PROPELLER CONTROL SYSTEM 7 Sheets-Sheet 2 Filed July 24, 1948 mukbae au P Lg H INVENTORS WILUAM P. R055"!!! MATTORNEY.

June 2, 1953 e. P. KNAPP ETAL 2,540,550

TURBINE PROPELLER CONTROL SYSTEM Filed July 24, 1948 7 Sheets-Sheet 3 TEMP MEN-R- 0 Halo 0 (BcorITRmi T Has 83 INVENTORJ GEORGE P. KNAPP W\\.\.\AM E-BURNS W\\ L\AM P. ROBNNS RAYMOND T. ZWACK June 2, 1953 e. P. KNAPP ETAL 2,540,550

TURBINE PROPELLER CONTROL SYSTEM Filed July 24, 1948 v 7 Sheets-Sheet 4 W CHANGE INVENTORY GEORGE P. KNAPP W\\.\.\AM E. auaus \NlLUAM P ROBB\NS RAYMOND T. ZWA K ATTORN EY.

June 2, 1953 G. P. KNAPP ETAL 2,640,550

TURBINE PROPELLER CONTROL SYSTEM Filed July 24, 1948 7 Sheets-Sheet 5 OVER "RIDE NORMAL June 2, T953 (5. P. KNAPP ETAL 2,640,550

I TURBINE PROPELLER CONTROL SYSTEM Filed July 24, 1948 7 Sheets-Sheet 6 CONSTANT PRESSURE FUEL M ATTORNEY.

June 2, 1953 KNAPP ETAL 2,640,550

TURBINE PROPELLER CONTROL SYSTEM Filed July 24, 1948 7 Sheets-Sheet 7 W096 REM.

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GEORGE P. KNAPP LL AM E- BURNS BY 2, [m

' fiATTORNEY.

Patented June 2, 1953 UNITED sures are NT ice 2,640,550 TURBINEPRORELLER"CONTROLSYSTEM fieorge Pi Knapp,"Verona-William E. Burns, Denville, and NVilliam P. Robbins'andfiaymond-T. zwack, CaldwelL N. 3., assignors to Curtiss- WrightiCorporation, a corporationofiDelaware 7 Application July'24, 1948,'Se1'ial'N0.40,476

41 Claims.

1 This invention-relates toprimemoverand load control and particularly'to a -system for'controlling within practical limits the operation ofa turblue-propeller combination comprising essentially a gas turbine including a compressor; and

means of reduction gearing to theturbine shaft. When used in aircraft during flight the comi p'ressor receives ambient air already boosted in pressure by ram action and delivers it at a'p-ressure of several atmospheres. to the combustion chamber .in excess of combustionrequirements,

Where part of it maintains combustion and the balance mixes withfthe highly heated products of combustion partially. to cool the combustion gases. This high temperature gas mixture constitutes the motive fluid which is directed under considerable pressure so as to operate thelturbine.

accordin to either the impulse or reaction principle, and then vented through'the turbine tailpipe to atmosphere. 'The temperature of the tailpipe gases or of the gases in the burners at predetermined or rated turbine'RaP. M. is representative of turbine torques'ince'for a given set of flight conditions the torque-tailpipe temperature relationship in a constantspeedgas turbine is for practical purposes linear. The tailpipe temperature or combustion chambertemperature also represents the'limit of permissible temperaturefor the turbine blades.

Under normal torque and temperature operatingconditions the turbine fuel supply andthe propeller pitch should be automatically con-- trolled by preset R. P. and .power controls operated by .the pilot. For. obtaining stable and precise automatic control, it has previously been proposed .toxcontrol turbo-propeller combinations in various coordinated Ways for example by varying the fuel input in accordance with turbine temperature and also by varying propeller-pitch according to theconj oint operation "of th'e'turbine governor'and'a manually-'corrtrolleddevice rep-resystems have serious disadvantages due for exampleto time lag in temperature control an-d ito insuificient or impropercoordination :of the various control factors tending to cause sluggishre- '=sponse,' or over-shooting resultin'g :in :unstable operation.-

One of the-most serious disadvantagesinaprior turbine control systems' of -'the computing type for aircraft is the fa'ilure-"to i'realize :and/or to limitfulloperating po'wer under va'rvi'ng fli'ght conditions.

For -example,"variable factors such as ambient temperature, ambient pressure and :ram pr essure, i.- e. the' pressure 'at the compressor intakedue to the ram "actionof the aircraft in flight, determine the tur'bin'e shaft torquel'tliat can be developed at a given turbine temperature, assuming that R. PI'M. is held constant. -'Also the torque is affectedby 'air -speed,=- decreasing' 'as theratio of'the ram-absolute pressure to ambient absolute pressure decreases, other factors being equal. Under conditions =such-as low altitude and low ambient temperature-(tailpipe temperatureand turbine H. 1 M. being constant) the torque available may exceed the mechanical limits of the-turbine-'gearing;etc.,whereas under high temperature orhigh altitudeconditions at the same R. P.-

and tailpipe temperature; the torque available "may be considerably less than the aforesaid limits. Accordingly*anycomputingtype of control-thatfailsproperly to takeinto consideration variable flight= con'ditio'n's *either limits the turbine to unnecessarily 'low"output 'or imposes a risk of structural damage to the turbineat maximum power.

Itis therefore an object "of the present invention to provide an improved turbo-propellercon- 'trol system "for aircraftthat is operable automat'ical-ly to expand or contract the power calibration of the pilots *power lever according to the existing flight condition's'soth'at maximum power can be realized-"for thelessfavoralc'ile conditionsflwithout exceeding the "turbine temperature limit, andso thatthe mechanical limits'of the' tu: "bine are not exceede'd'for mor'e' fav'orable conditions, withoutlimiting the actual or elfective'stroketof'the'powerlever. I

Another object'ofithe invention is tofprovide an improved -system of "the above character for obtaining precise control "of turbine R. P. .M., rapid and precise control of turbine temperature at or 'near maximum power "and 'for precluding objectionable transients of R. P. turbine'temsentin power-setting. Manyofthese'prior'art AI IT jfl "of" the inventionis *to p e operating schedules for means for obtaining improved operation under low airspeed or low power conditions, including control of propeller thrust at low power with means for obtaining substantially zero thrust; selective reverse pitch for aerodynamic braking at landing airspeeds; feathering, and means for obtaining the minimum torque angle during starting.

The invention will be more fully set forth in the following description referring to the accompanying drawings, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Referring to the drawings:

Fig. l is an exposed plan view of the forward part of an airplane fuselage illustrating the general arrangement of a turbo-propeller combination together with associated control equipment to which the present invention is applicable;

Fig. 2 is an enlarged plan view of the pilot's control quadrant shown in Fig. 1 indicating various control lever positions that may be selected by the pilot;

Fig. 3 is a block schematic diagram illustrating the general relationship between the principal component parts of the control system embodying the present invention;

Fig. 4 is a diagrammatic illustration of an electrical system for computin available turbine shaft torque and for determining torque limits;

Fig. 5 is an elevational view mainly in section of mechanical control equipment associated with various parts including the turbine proper, the propeller pitch changing mechanism and the computing circuits;

Fig. 6 is an enlarged detail view of the dump valve cam structure shown in Fig. 5;

Fig. 7 is an enlarged view illustrating in perspective details of adjustable pitch controlling means shown in Fig. 5;

Fig. 8 is a simplified lay-out of the differential mechanism shown in Fig. 5 for illustrating the essential operation thereof;

Fig. 9 is a diagrammatic illustration of another part of the computing and control system including tailpipe or combustion chamber temperature responsive means for effecting temperature correction oi computed torque;

Fig. 10 is a diagrammatic illustration of another part of the computing and control system including the governor control circuit for regulating tubine R. P. M.;

Figs. 11 and 12 graphically illustrate different the turbo-propeller system;

Fig. 13 graphically illustrates certain operating characteristics of a variable pitch propeller; and

Fig. 14 graphically illustrates turbine operating characteristics.

A turbine-propeller combination of the type to which the present invention is applicable is shown generally in Fig. 1 which represents a single engine installation wherein a gas turbine I is suitably mounted within the forward part of an aircraft fuselage 2 (shown exposed) for driving a variable pitch porpeller 3. The turbine which receives fuel from a fuel line 4 is connected to the propeller through speed reducing means such as reduction gearing (not shown), and the propeller pitch changing mechanism (also not shown) can be considered as located within the spinner 5. The pilots cockpit 6 as viewed from above is located to the rear of the turbine and includes an instrument panel gen erally indicated at l and the pilots control quadrant 8.

The control quadrant is shown as it would be viewed by the pilot in more detail by Fig. 2 and includes according to the present invention three manually operable levers, namely a power lever 9 calibrated in percentage of available power and operable to the positions of full," zero and reverse power, a condition lever 10 operable to any one of five positions hereinafter described for providing flexible and emcient operation under varying flight and ground conditions and a fuel lever H for manual control of the turbine fuel line 4. The control quadrant includes all the preset or adjustable controls that are required by the pilot for practicing the present invention. The feel of the levers can be adjusted by means including a knob l2 which, in well known manner, regulates the frictional drag on the levers coincidentally.

In the arrangement shown, the fuel lever is connected by a flexible cable 13 to a fuel valve It in the line 4 and the power and condition levers are connected by flexible cables It and I5 to electrical computing apparatus hereinafter described generally indicated at H. The computing apparatus is also electrically connected by conducting cables l8 and [9 to temperature responsive devices 28 and 2! located respectively on a wing of the aircraft and in the turbine tailpipe for indicating ambient air and tailpipe temperatures. Also the computer is connected by an electric cable 22 to a plurality of pressure responsive devices and a servoinotor located in the turbine control compartment generally indicated at 23.

The turbine control compartment is specifically shown by Fig. 5 and includes the turbine governor control, propeller pitch control, and the aforesaid pressure responsive devices subject to both ambient atmospheric pressure and ram pressure. The ram pressure is obtained by means of an open-end tube 24 located in the compressor intake passage 25 so that it is subject to the ram action of the air during flight. The means responsive to ambient pressure may be connected by a line 26 to any stable source of ambient pressure such as the cockpit, assuming the cockpit is not pressurized. The turbine control compartment is also provided .with fluid connections 28 and 29 (Fig. 1) representing respectively the pressure signal from the turbine torque meter and the governor oil intake, and also connection 30 from the governor for controlling the fuel pump 31 as will be hereinafter described in connection with Figs. 5 and 10. The fuel pump 3! although used in practice is not disclosed in detail herein, it being sufficient for present purposes to show a valve for regulating flow of fuel under constant pressure. The turbine control compartment also is connected by flexible cables 32 and 33 with devices 34 and 35 associated with the pitch changing mechanism for indicating the pitch angle and for effecting pitch change respectively. The device 35 may comprise known means for regulating the rate of pitch change of any preferred type. For instance, the apparatus shown in Mergen et a1. patent application Serial No. 143,636, filed February 11, 1950, may be used.

The above description of Fig. l is intended mainly to indicate the general mechanical arrangement of the principal components of a turbo-propeller powered aircraft and its power control system according to the practice of the present invention, and to illustrate the manner fuel-air ratio.

in which the computingandcontrol 'aDIJBJZatus-Z' ;,associated: with" the propeller, the :turbine ,proper ,and the pilots controlquadrant. .The functional relationship ofessential componentparts of the control system is diagrammatically illustrated by'Fig. 3 wherein the propellerpitch (.13) chan ing mechanism and theturbine fuel line 4 are indicated as controlled according to theposi- .tioning of the torque -(power) and condition levers 9- and 10 respectively and alsoaccording to the output of computing apparatus with respect to.the instant indicatedpropeller pitch. The

computeris controlled primarily by inputs in- .volving severalfactors, namely ambient temperature, ambient pressure and ram pressure. As indicated, the adjustment of fuelfiow at thegov- ,ernor controlled automatic fuel valve Va controls turbineR. P. M. whereas propeller pitch is nor- ,mally adjusted according to power. lever setting,

torque computer output,.and under certain conditions tailpipe temperature correction of the torque computer output.

In the foregoing, the terms power? and {torque havev been used. Power fromthe tur- .bine is a function of turbine RP. M. andtorque delivered in the propeller shaft. .Power is also a function of thrust delivered by the propeller. Torque and thrust are both force components of power produced by-theturbine. As was pointed out in earlier patent application Serial No.

694,398 filed August 31, 1946, there. areseveral alternative power components or manifestations utilized for turbine control purposes, such as turbine temperature, torque, thrust, fuel flow and the instant application are the force components, namely, torque delivered by the turbine to the propeller, or thrust produced by the propeller. The term force components of power used herein thus connotes either-torque orthrust.

If now it be assumed that R. P.-M. is controlled accordingto the propeller blade angle, it will be seen from Fig. 13 that because of the reversal of slope-that always occurs at some point in all curvesof propeller torque as a function of the blade angle, the sense of blade angle control reverses at some blade angle, usually a low, angle, and always at a point corresponding to a low turbinetemperature. Furthermore, asgthe re- .versal point is approached, the control becomes increasingly sluggish and insensitive in action by .reason ofthe comparatively flat slope of thecurve .-near this point. Although turbine temperature is low as above noted at the reversalpoint sothat temperature controlcan-be safely relinquished at that part of the control range, R. .P.:M.,control can never be relinquished since it is a primary requirement-that thercontrol be capable of maintaining maximum R. P. M. for allpowercondi- .tions.

Ina fuel flow control of R. 'P. M. ,onthe other hand there is no such reversal of control sense nor is the. control range insensitive. in part.v Fur-- .thermore, while turbine temperatureand ,R. ,P.

M. are theprinci-pal factors ,toJbe. control ed from the standpoint of protection of the -.tl.11.'b ine.,pro-v peller thrust is a factor of great importance 'to The particular components of power which are useful as control. components in are avoided. A change in turbine R. ,P. M. ,re-

, quires alargechange in the. kinetic energy of the rotor assembly which .must .be supplied ,by changes in turbine fuel flow, or aircraftkinetic energy or both. In a blade angle control .,of R. P. M., an RF. M. change is instigatedtby changingthe turbineload by varyingthe propeller pitch in a direction such that the.incr.e mental torque change is opposite in sense to the incremental R..P. change; for instance to increase R. P. M. the propeller, pitch must bedecreased which in turn decreases torque and thrust and decreases the aircraft kinetic energy. Since a demand for more R. P. M. is ordinarily associated with a demand formore torque, a control of this type is undesirable. ,Onthe otherhand a fuel flow type governor acts directly to control theytorque; output of the turbine, so-gthat there is no primary effecton thrust and'the small thrust change that does occur is in the proper sense. Although the increase in R. P. M. is accompanied by a transient increase in turbine temperature, this ,is not usually objectionable since the ,temperature is ordinarily relatively low when R.-P. M. is below its maximum value.

As indicated in the diagram of Fig.3, the ,coordinated control of this invention includes -a temperature corrected computer output representing maximum available torque,,and,this output is modified by the desired percentage of torque selected bythe power lever of the control quadrant which is usedin conjunction withithe turbine torque actually developed for effecting propellerbitch change. The condition lever of the control quadrant is :operated in ,a manner hereinafter described to insure proper coordination of torque, fuel valve position, R.,P. M. and. propeller pitch for various conditions, such as starting and stoppin ground idle, cruising-and intercepts of the characteristic curves varyv so radically that the torque obtainable at maximum turbine temperature varies over a range of approximately 4 to l, as graphically shown by Fig. 14. Thefavorable condition illustrated for high torqueis represented by sea level altitude and -60 F. ambient temperature and the unfavorable conditlon is representedby'about 35,000 feet altitude and ,60 F. ambient temperature, the airspeed factor beingsimilar inboth cases. In

;conventional operation it would therefore benec- ,essaryforthe pilot either to limit the'rangeof the torque control lever to the power obtainable under the least favorable flight. conditions or to keepconstantwatch of the turbine temperature in order to avoid operation at dangerous overtemperatures under more favorable flightconditions. Accordinglyit willbeseenzthat the operating range of the torque or power lever must'fit'the current flightcondition so:,that:the actual .power range obtained by the power lever is compressed or expanded as requiredinorder to obtain high operating efficiency and optimumturbine per- The power ;lever,.w,ill thus phave.:.an

desired percentage of currently available power.

To this end, electrical quantities are derived to represent the variable flight conditions for input to electrical computing apparatus, the output of which represents the torque which would be obtained at maximum permissible temperature and R. P. M. for the current fiight condition. This output is proportioned according to operation of the pilots power lever for selecting the percentage of power desired for the maneuver at hand and the resulting electrical quantity is used for controlling the propeller pitch and hence the shaft torque.

Torque computer The torque available at maximum R. P. M. and

precletermined maximum turbine temperature within the range of ambient temperature and pressure conditions and air-speeds commonly encountered in flight can be expressed by the following empirical formula:

1K3( tT.=)

(Equation 1) where The electrical torque computer in one embodiment of the present invention shown essentially in Fig. 4 utilizes a servo and potentiometer system including pressure responsive devices for deriving control potentials representing Po and (PPa) and temperature responsive devices for deriving other control potentials representing Ta and a correction turbine temperature, the maximum permissible turbine temperature being set by way of example at 1250 F. This computer, which is operatively related to the turbo-propeller control system as hereinafter described, functions according to the above formula to make available the maximum permissible torque over the complete range of flight conditions.

Referring particularly to Fig. 4. a signal voltage is derived according to the ambient temperature To. by means of a Wheatstone type resistance bridge 40 including in one leg thereof a resistance element 4! having a suitable resistance-temperature characteristic. This element represents the Ta pickup and may be in the form of a flush mounting resistor bulb installed at any suitable point in the aircraft wing or fuselage where a free air stream exists. The Ta. pickup unit is indicated for example on the wing at 2!) in Fig. l. The bridge 40 is energized from an alternating current source Ear: through a transformer 42, the resistances 43, 44 and 45 in the other legs of the bridge being proportioned in relation to the resistance 4! so that the voltage across the bridge junctions 4B and 41 is proportional to the difference between a predetermined reference temperature and the absolute ambient temperature, namely (K4Ta). This voltage represents the Ta signal which is amplified by amplifier 48 for energizing through an output transformer 49 a pair of potentiometers 50 and that are controlled according to the values of P0 and (PO-Pa) respectively. The amplified signal which is proportional to (lift-Ta) also energizes as inv dicated another potentiometer of the computer hereinafter described.

The potentiometer 5| is in addition energized from the source EEC through transformer 52 for representing a constant factor so that the summed voltage across potentiometer 5! from both transformer sources 49 and 52 represents (Ta-K5). For the purpose of simplifying the description of operation of the computer, the various constants involved in a particular turbine control design are omitted, it being sufiicient to state that the computer operates essentially according to the values of the variables Ta, Pa. and (P0-Pa) for determining available turbine torque at maximum permissible temperature.

Suitable pressure responsive means, such as sylphon metallic bellows control slider contacts of the potentiometers 50 and 5| for deriving control voltages in the following manner: a voltage e1 is derived from potentiometer 56 by the slider contact 53 that is mechanically connected as indicated to a pair of metallic bellows Ella and 501) so as to be operable jointly thereby. The bellows 50a is subjected to ram gage pressure from the tube 24 located in the compressor intake passage, Fig. l, and the evacuated bellows 59b is subjected to the ambient or barometric pressure so that the additive bellows movement represents the ram absolute pressure P0. Since the potentiometer 50 is energized by a voltage corresponding to (K4Ta), the derived voltage represents P0 (K4Ta). The voltage e1 so derived at contact 53 is in additive relation to the potential derived at contact 56 of potentiometer 5| by reason of a conductor 54 connecting the contact 53 and the mid-tap of a resistance 55 in parallel with potentiometer 5 I. This potentiometer is, as above described, energized by a voltage representing (Ta-K5). Accordingly the derived voltage 62 from potentiometer 5! at the slider contact 56 which is also operable as indicated by a metallic bellows 51a subject to ram gage pressure '(P0Pa) from the tube 24, Fig. 1 represents K2 (Po-Pa) (Ta-K5) The voltage (32+61) is further combined additively with voltage e3 which may be phased in either direction asrequired and which voltage is derived from the operation of a tailpipe tempera ture correction potentiometer 51 that is energized from the source E86 through a transformer 58. A conductor 59 connects the contact 56 of potentiometer 5! with a mid-tap of resistance 60 in parallel with potentiometer 51 so that any corrective action from the tailpipe temperature measuring equipment and motor hereinafter more fully described causes a change in voltage e3. This control is of the integral type so that the position of slider 6i and consequently voltage 63 bears no fixed relationship to existing turbine temperature. Its speed of correction is, however, related to temperature error.

Accordingly, the total corrected voltage can be represented by ei+ez ea, namely,

which corresponds to the numerator of Equation 1 after temperature correction. This summed voltage is amplified by the amplifier 62 to a voltage indicated \as e4 which is impressed across a portion 63a of a potentiometer 63. A zero reference terminal of potentiometer 50 is connected by conductor 64 to the zero power terflit minal B6 of potentiometer 63 and the opposite or be used directly for controlling the pitch changing mechanism of the propeller.

Referring particularly to Figs. to 8 inclusive, there is disclosed a differential mechanism suitable for controlling propeller pitch in the above manner. Referring first to the simplified layout of Fig. 8, the servo-motor BI is connected through a pinion gear 9| to the ring gear 92 of a differential 93, one sun gear 94 of which is connected through a pinion 95 and rack 96 to a pressure responsive device 91. This device may comprise a spring biased piston 98 connected to the rack 96 and mounted within a cylinder 99 that has a fluid connection I09 with the turbine torque meter (not shown). Variations in turbine torque cause'through the torquemeter corresponding hydraulic pressure changes in the cylinder 99 to operate the piston, rack and sun gear 94 of the differential 93 accordingly. The differential includes the planet gears 92a. and 92b arranged in conventional manner on the ring gear for interconnecting the sun gears.

The other sun gear IOI is connected directly by shaft I92 to a sun gear I03 of a second differential I04, the ring gear I05 of which is normally held in fixed position by a gear sector I06 that is biased against a fixed stop Ifl'I by a preloading spring I08, Fig. 7. Thus with the ring gear stationary, differential I04 serves merely to transmit motion. The function of the gear sector I06 will be described later since it comes into play only when the pitch is reduced to below zero or flat pitch or to negative pitch.

Accordingly for normal operation when the system is under torque control the differential 93 only, functions as such and the sun gear I93 of differential I04 drives the other sun gear I99 at the same rate as the output of differential 93 through the planet gears 15a and I 05!) connected to the ring gear. The sun gear IDS is connected by a flexible shaft 33 to a pitch change control device 35, Fig. 1, of the propeller pitch changing mechanism. Known means such as electrical, hydraulic or mechanical systems for controlling the rate of propeller pitch change can be used in combination with the aforesaid control device 35 for increasing or decreasing the blade pitch angle at a variable rate according to whether increased or decreased torque is called for. Copending applications of Cush-man, Serial No. 771,022 filed August 28, 1947; Mergen et al., Serial No. 34,984 filed June 24, 1948; and 'I'iedeman et al., Serial No. 776,956 filed September 30, 1947, now U. S. Patent No. 2,620,887, issued December 9, 1952, all show propellers embodying a rate of pitch change control appropriate for use in a control system of the type herein disclosed. These applications are in addition to the previously mentioned Mergen et al. application SerialNo. 143,636 filed February 11, 1950.

It will therefore be seen from Fig. 8 that the differential 93 functions to compare the torque called for by the servo-motor 8i and the cur rently existing turbine torque indicated by the torquemeter device 91 so that the desired torque correction, either positive or negative, is represented by rotation of the sun gear IN and corresponding rotation of the pitch rate control shaft 33. As the turbine torque changes in the direction of the indicated correction, the torquemeter pressure acting through piston 98 tends to adjust the differential 63 (and shaft 33) in a direc tion to neutralize the device 35 (which has previously been moved by servo-motor BI) and thus reduce the rate of pitch change as the tur- 12 bine torque approaches the desired torque. The extent and sense of the rotation of the shaft 33 therefore normally is a measure of the rate and sense of change of propeller pitch required to adjust the turbine torque so that it approaches the torque called for in a manner that when combined with a properly designed governor minimizes overshooting or hunting.

The above described operation of the computer and control system is limited to a normal cruising condition, referred to on the condition scale, Fig. 2, as normal operation, wherein no emergency conditions are ordinarily involved, and wherein the pilot obtains complete turbo-propeller control by merely moving the power lever 9 to the power setting desired. During this operation the turbine speed may be varied according to a certain schedule which coordinates power and R. P. M. for example by governor apparatus, Figs. 5 and 10, arranged to control the turbine fuel supply. The maintenance of maximum R. P. M. for a given permissible tailpipe temperature is essential for eificient operation since decrease of turbine R. P. M. rapidly reduced the power output for a given tailpipe temperature. In the present invention no R. P. M. bias of the computer is required since with the particular schedule of power and R. P. M. employed the coordinated control insures that maximum permissible torque (temperature) is never exceeded.

The T9. signal amplifier 48 and the computer amplifier 62 of Fig. 4 are the only ones in which the gain affects the output of the computer. These amplifiers therefore include large amounts of inverse feed-back so that their gain becomes substantially independent of tube characteristics according to established practice.

The special conditions requiring feather-mg" (essentially 90 pitch), flat pitch (essentially 0 pitch) and reverse pitch are represented by settings of the condition lever II]. For example, when the pilot wishes to feather a propeller of a multiple-engine aircraft during flight because of engine trouble, the condition lever for the engine in question is moved to cut-off, thereby impressing full torque voltage (computed) across the balancing circuit and cutting ofi the fuel supply by closing fuel valve I6 to cause the blade control mechanism to run toward feather." Specifically, when the grounded condition-lever contact 88 is moved to engage the contact 8801 at the cut-off position, the power lever contact II connected to condition contacts 88d and 88e is disconnected from the circuit and the full torque voltage as of potentiometer 13a is impressed by way of conductor I la and common ground across the balancing circuit including the servo amplifier 86 and motor 8I. Since the balancing slider 89 is presumably in an intermediate torque position, the full torque voltage e6 is greater than the balancing potentiometer voltage c7 and the difference voltage energizes the servomotor iii to move the slider contact toward the left, or maximum torque position where the voltage difference in the balancing circuit is reduced to zero and the motor deenergized. Since, with the fuel now cut off, no energy is supplied by the turbine to sustain torque and R. P. M., the turbine decelerates, causing the torque measured by the torquemeter to decrease, and thereby instituting an increase pitch signal through differentials 93 and I04 and rate control shaft 33. This operation results in maximum permissible decelerating torque being applied to the turbine, and propeller feathering is thus accomplished in the .513 minimum .apermissible-time. When the .feather angle is reached,.-pitch change ceases. byreason of the action of a-mechanical-stop in the propeller set: for the. correct -angle. To summarize; when the blade pitch is positivethe. difierentialllM, Fig. 8, functions-merely-to transmit to the rate control shaft 33 the pitch correction indicatedby the output of differential.93. .(shaft .162) jointly controlled bythe-servomotor BI and torquemotor pressure, and the. differential. output represents thezrate of pitch change required .The feather control is therefore "directly controlled according to the "voltage es repr-esenting 1 l00%-.to,rque. Since the application of fulbtorquecalledforby servomotorrBI with fuel cut off results in-..full torque being sustained. by increasing propeller pitch; itwill "be apparent'that the blade. is changed to --feat-her at: the maximum rate-permissible.

:.It is essential that the propeller'beadjusted to azminimurmtorquez angle during starting to keep starter torque requirements to a: minimum. It is also necessary to adjust the propeller to essentially the'aero thrust'angle during ground idling,.and during normal operation and take- .off. and landing when the power lever is at the zeropower position. This angle is fortunately essentially .the..same as the minimumor zero torqueangle. .It is further required that the controlbesafeguarded from inadvertently running vthe. 'Jro1'Jeller- -to reverse pitch during taxiing or ground. idling.operationswhen the control calls for zero torque a'ndthere is no blade pitch at which zero torque can be obtained (see Fig. 13).

According to the invention in its present form the propeller. is adjusted to flat pitch when the condition lever is set at start and .ground idle. However,. at fstart with no shaft rotation the shaft torque and the computed torque voltage are .both zero and therefore if the engine was stopped with theblade feathered no fdecrease pitch signal is available immediately for. bringing the propeller to flat pitch. For the purpose of providing a decrease pitch signaL'. the condition lever at start provides a signalequi'valentto calling for asmallamount of. negative power. This partof the control functions .asfollows: when thecondition lever contactiiit is at. either the startf. orfiground. idle...setting,..the servo amplifier. 85. is connected .through. the common groundandconductor I3I to ataplISIa. on a potentiometer section 1312 of the balancingpoten- .tiometer l3= defined by. terminals'll and 130. This potentiometer section 13b. is energized from -.the A. C. reference'source Eac through a transformer I9 and its voltage has opposite phaserelation with respect-toterminalTi -to the voltage across potentiometer sectionlsm Thereforathe voltage tap I3I-a on the potentiometerlw could .be considered as representing a small-negative torque, such as 2%. for example ofthe..total potentiometer voltaga ior providing asignal for decreasing. propeller pitch. servomotor 8| runsthe difierential control inthe Y direction of decreasing. pitch.

Accordingly the i Pz'tch over-ridacont rol It will be noted, referring to the propeller characteristic curvesof Fig'."l3, that it is .impossible at sea *torque iscalled for, the propeller control unless,

ream compensated tends to reversethe blade. angle to full reverse. in an attempt. to;- satisfy; the; zero torquer'signal. '.".That is, unless the pitch. control by torque is overridden, thepositive torque .signal fromthe-torquemeter in combination with a zero. or aneg'ative. torque signal from the servomotori 8| would; tend. toimake, the. blade angle negative. I This. in .turn would further increase theotorquemeterpressure with the result that control is .lostand .the propeller is rapidly,,operated to a position offull reverse pitch.

'The control for. v.adjustingi the? blade .to' fiat pitch without overshootifunctions .in. the following manner taking for i11ustrati0n,.the ground starting conditiomllthe .pitch is reduced from feather. (assumingthattheeng e Was previously stopped. .'.with the. propeller..,feathered) to fiat pitch.asthelservomotor 8L moves theislider toward.thehalanceposition attap" I3Ia.'. At the .point of. zero pitchan override mechanism comes into. play tor-hold. the propeller atsubstantially .fiat .pltC1l. ..ThlS overwride. mechanism involves viblade angle follow 1 up. control as. distinguished 'fromthe previously... described torque control and is specificallvillustrated bvFig. 7 taken in conv.nt-ictionwith.Figs. Sand 8. l The. pitch indicatin device .34., Fig. 1., is-aspreviously stated connectedthrough aflex'ible cable" 32 tooperatethe gear. sector I06, Fig; 7.. A sh'ait..32bwhich constitutes. .an. extension of cable I 32 has mounted thereon the gearsector. I05. having a hub. lliBa .which is free. to. rotate ontheshait 32b. Apitch index fingers I IlLthat isrp'os'itioriedbythe shaft is .adjustably. connectedlthereto by .means of a Vernier adjusting .deviceiIllpincluding a collar II Iasecuredtotheshaft and an-adjus'table collar IIIb that is rotatable..-on,theshaft. The index finger. III]. is. secured, tothe collar. I I lb. and

this collar is secured'with respect. to. collar I I la by. means of. an;intermediate..vernier collar I.I Ic also rotatable ion the. shaft. f The; .three, collars are suitably clamped togetherby. means on the shaft and are in .toothed engagement as indicated the Vernier. collar. II to having for example 2.9 teethat one side for engaging asimilarnum- :ber. otteeth .on.collar I.I Ia1and30 teeth atlthe other sideforengaginga similar number of. teeth on collar- II lb .therebylprovidingin well known ,n-ranner a fineadiustment of. the. index finger. I II with respect, to .the. shaft. .The. adjustment may pm at .all. values ofpositive pitch.

Accordingly when the. blade. pitch is. indicated .as:less.than zero, theindex .I I-Oengaging the. .pin

IIItb .has rotatedthegearsector I06 and hence the-ring gean-Iil5 of.the diiierential Hi l a corre- ..spond1ng...amount.. .The 7 difierential. I94 1 now v.iunctions, assuch and thedifferentialBS serves merely to transmit motion from the servomotor 8I...since. the'torquemeter fishy-passed substantially at and. below zero. pitch as: presently-explained. :Normally when @the blade angle is positive-.andthe system issunderttorque control. referringnow to Figs. -7'and 8,-the differential I-M serves. .merelyas part of the transmission shaft .while the diiterential.r 93- is jointly controlled by .;the servomotor. and. torquemeter to. produce a differential rate. control quantity. When however thegearsectorIlifi. is rotated in responseto areverse pitch indication of. shaft.32, the arrangement is such that the-.ringgear. I05 of differential I 0.4 isrotated in adirection to. counterbalance. or: neutralize; the pr'eviousdecrease. pitch signal from shaft I02 and to close the rate control mechanism 35 so that the blade remains at a slight negative angle, i. e. substantially flat pitch. In other words, a negative departure of pitch angle from zero causes the differential I04 to offset the decrease pitch movement of shaft I02 so that the propeller control represented by the output shaft 33 is stabilized at the small negative blade angle indicated by the position of balancing slider 80, i. e. at the essentially minimum torque position represented by voltage tap I3Ia of the balancing potentiometer 13b.

In order to substitute blade angle control for torque control when the blade angle is decreased below flat pitch, the torquemeter indication at differential 93 is eliminated so that the position of servomotor 8| indicates the blade angle desired. To this end, a bypass or dump valve I32, Fig. 8, is provided for returning the torquemeter oil to the sump. The dump valve I32 is related to a cam I33 controlled by the pitch indicator shaft 32b and is actuated by the pitch indicator cam when the pitch angle becomes less than zero thus preventing the build-up of oil pressure and consequent deflection of the torquemeter piston 98. Accordingly, the piston spring returns the rack 96 to its initial position where it, together with sun gear 94 is held stationary. In the specific arrangement shown in Figs. 5 and 6, the cam I33 has an elongated slot I3 3a arranged eccentrically with respect to the shaft 321) and the plunger I34 of the dump valve is provided with a follower-roller I34a that rides in the cam slot. The cam I33 is adjustably secured in a suitable manner to the shaft 321:, such as for example by a vernier device I33c similar to that described and shown in Fig. 7 whereby the cam can be precisely adjusted for operating the dump valve with respect to zero pitch.

Reverse pitch under power is possible only at the take-01f and landing setting of the condition lever. This setting is always used for example during landing maneuvers since full reverse thrust under full power may be rapidly required for quick braking. In this position the condition lever contact H9 which is connected by conductor 9a to the negative side of the power lever potentiometer 63, engages a fixed contact I I9b connected by conductor I I90 to the terminal 130 of the balancing potentiometer 1312. Therefore in the take-off and landing position the potentiometer 63b which in this case may be calibrated according to negative blade angle is energized directly by a fixed voltage from the balancing potentiometer 1311 representing a negative blade angle limit. Thus in this condition lever position the power lever may be used throughout the entire power range for torque control in the case of forward thrust, and for blade angle (13) control in the case of reverse thrust.

Since the power lever slider H is now connected (through condition lever contact 88) in the balancing circuit, the servomotor 81 is effec tive to position the balancing slider 80, and hence the pitch controlling differential apparatus accordin to the position of the power lever on the 3 potentiometer 63b thereby providing reverse thrust control for the aircraft throughout the full reverse pitch range of the propeller. During reverse pitch operation when the system is under blade angle control, the pitch angle is a direct function of the power lever setting by reason of the follow-up action of the differential mechanism above described. That is, as the servomotor 'BI moves slider in the direction of reverse pitch beyond the minimum torque position to a position corresponding to the negative blade angle setting of the power lever, the propeller control becomes one of the blade angle follow-up type wherein the blade angle is a function of the setting of servomotor 8!. This is because the torquemeter is ineffective throughout reverse pitch control and the pitch indicator control of differential I04 tends to close the rate mechanism 35 so as to reduce the rate of pitch change called for by the servomotor 8| as the actual itch approaches the pitch called for by the power lever setting on ,3 potentiometer 63b. As the blade angle is decreased below flat pitch, the propeller shaft torque becomes positive and eventually increases under certain flight conditions to the full rating of the power plant, thereby providing maximum reverse thrust for braking. The full reverse pitch stop in the propeller is so chosen that under no condition is the maximum rating of the power plant exceeded.

Torque over-ride control As previously indicated in connection with Fig. 14, it is possible under certain flight conditions to obtain turbine torques considerably in excess of the rating of the power plant before the maximum permissible tailpipe or turbine temperature is reached and it is therefore necessary to have a maximum torque limiting device so that the maximum torque which the turbine reduction gearing system can safely withstand will not be exceeded.

Normally the corrected and amplified computed torque voltage from amplifier 62 energizes the power potentiometer 63a through the discriminator relay DR. The function of relay DR is to switch the power potentiometer 53a from the computer amplifier 62 under conditions where abnormally high torque could be obtained to a reference source of potential, i. e. the potentiometer portion 13a energized by voltage es (the denominator value of Equation 1) representing maximum permissible torque, thereby overriding or taking the control away from the computer.

For this purpose the relay DR comprises a pair of movable contacts 68c and 68b (contact 681) shown inactiv in Fig. 4) gang-operated as indicated and spring biased to one position and operated by the relay coil 680 to the other position, these contact positions corresponding to the normal and over-riding control respectively. The coil 680 is energized from a discriminator circuit II2 that functions to compare the computed torque value with a permissible or reference torque value. The input to the discriminator circuit IIZ represents the positive or negative difference between the aforesaid torque reference voltage c6 and the computed torque voltage, i. e. the output of amplifier 62. The voltage er although variable in magnitude between wide limits as flight conditions change always corresponds to maximum permissible torque. Specifically, the nput of a conventional summing amplifier II 3 18 connected by conductor H4 to the output of the computer amplifier B2, and by conductor H5 to the high voltage terminal of reference potentiometer 13c, so that in effect the voltages across potentiometers 63a and 13a are compared at the ampliizgr. d'lhe resultant amplified output energizes e iscriminator circ i former "6 u t through trans The discriminator circuit per so can be 0 suitable design capable of sensing a positifre r a i negative difference of computed torque voltage with respect to the reference voltage for energizing the DR. relay coil accordingly. 'In the arrangement shown, the relay coil is energized only when an over-ride condition obtains. If. for example, the computed torque voltage does not exceed the reference voltage, the coil 680 is deenergized and contacts 58a and 69 are spring biased to close asshown forconnecting the computer to the control system. If on the other hand thecomputed torque voltage exceeds the reference voltage,'the relay coil is energized to'operate contact-68a in the opposite direction to engage contact H1 and transfer the control from the computer to the reference torque potentiometer 73a. The power setting potentiometer 63ais now "directly connected to .the es reference voltage "source by conductor 61, relay contacts 68a and II! and conductors H8 and H5 so that the percentage of torquecalled for .(voltage e5). by the :power lever setting is within safe limits of turbine capacity.

.Briefly,'the discriminator circuit II2 functions to produce. a D. C. voltage output that is propor- 'atransformer I27. A rectifier tube IE8 is controlled according to the sensing operation of "tubes .I24 and I25 for energizing therelay coil 68c and is related thereto in the following manner: the grid of rectifier tube I28 is connected to the cathode of tube I25, andthe cathode of tube I24 is connected to a negative biasing source I29.

'The plate circuit of rectifier tube I28 includes conductor I30, coil 58c and aD. C..plate voltage source as indicated. Accordingly, for normal operation whence is greater than e4, that is, when torque computed is within .permissible "limits, the grid and plate of tube I24 are concurrently positive so that this tube-is conductive,

and tube I25 is non-conductiveby reason of its .out-of-phase grid voltage. This conducting .ac-

tion or tube $24 increases the potentialdrop across the cathode resistance so that the resulting lower potential atjunction I126 together with the negative bias I29 .places a strongnegative bias on thegrid of rectifier tube I25. "There'foreffor this condition the rectifier tube does not conduct and the relay coil is deenergized.

On the other hand when 641s greater than cs, that is when computed-torque isgreater thanthe permissible maximumtorque of .14,000jlb..ft. for

example, the tube I24 is out off and the tube I25 is made conducting with the result that the positivepotential at the cathode of tube I25 overcomes the negative bias TIES and'makes positive the grid of the rectifier tube. This tube therefore'becomes conducting so that the relaycoil 6.80 is 'energized'tooperate the relayJDP. to the overrideposition.

The circuit details are soselected that relay pick-up anddrop-out'do not occurso-close t0- gether with respectto the difference between'the computed and constant-torque-desired.signals. as to cause instability, .cr shiftingback. and forth of alternate control. signal source .in the region where the difierencebetween these twosignal voltages j is small.

;In summary: when the flight conditionis'are favorablefor developing high turbine torqueas "graphically-shown by Fig. 14, the output-voltage of the computer, if larger than the reference torque voltage, causes "the discriminator-device to over-ride the computer thus protecting the turbine from the 'efiects'of excess developed torque. Since the over-riding control operates independently of the position of the power lever, it will .be apparent that excess torque in the high-power range can beanticipatedwhen the power'lever is set for low-power demands.

Operation of the'power lever selectsa' certain percentage of torque, either of computed available torque as in normal operation or of maxi- .mum allowed torque as where the computer is over-ridden. It should be noted that the Voltage derived "from power potentiometer {53a simply represents a desired percentage times the computed available torque (although the actual'voltage corresponding toa given percentage of maximum currently available torque is a variable), whereas whatever voltage happens to be impressed across the reference torque potentiometer 13a represents the maximum allowed torque in lb.-'ft. due to the gear limit. Accordingly in the override position, the power lever is actually selecting the desired fraction of adefinite amount of torque in lb.-.ft.

Temperature correction control Although the operation of the computer-is-reasonably accurate in thatin general an equation may be heuristically determined as in the example heretofore cited which will establish a close corelation between the computed torque and the actual currently available turbine torque, it is considered good practice to incorporate a correcting or over-riding control based on actual measured turbine temperature which acts when necessary to correct the indications .of the computer so that maximum permissible turbine temperature is always obtainable and is never exceeded except during brief transient periods. An over-riding control of this character will therefore function to compensate for anticipated aging of the turbine whereby, because of erosion or dust collection on the compressor blades or other causes, the power available from the turbine is reducedas engine time accumulates. This control also provides a limited amount of protection against sudden malfunctioning of'the powerplant such as that whichmight be due to battle damage. The action of the temperature over-ride is such that at thefull power setting of the power lever where maximum performance is desired, the output of the computer isincreased or decreasedas necessary toobtain maximum permissible turbine tailpipe .0]? burner temperature.

The turbine temperatureresponsive means and the temperature over-ride control associated therewith are illustrated in Figs. 4. and 9 taken together. As previously stated, the temperature correction potentiometer .5? of Fig. 4 isadjusted 19 calling for maximum torque, the computed torque is increased so that the maximum permissible torque is available. The control functions so that the temperature over-ride subtracts from the indication of the computer at all power lever positions, thereby controlling over-temperature in g the event of malfunctioning of the power plant.

The control of the tailpipe temperature motor foradjusting the temperature correction potentiometer 51 of Fig. 4 is illustrated by Fig. 9

wherein four tailpipe thermocouples I35 of suitable time response characteristic are connected in parallel-series arrangement to constitute the turbine temperature pick-up. This arrangement provides an averaged value of tailpipe (or burn- .er) temperature as well as increased voltage output, and increased safety factor in case of failure of a thermocouple. The D. C. thermocouple potential is converted into an A. C. signal byv means of a saturable reactor circuit I36 which includes cold junction compensation. The A. C. signal obtained is in turn amplified for operating the reversible tailpipe temperature motor I 31 through a range corresponding to the time integral of the temperature departure from the pre-set value. This motor in turn corrects the computed torque voltage, Fig. 4, so as to vary the turbine torque and hence through action of the governor which regulates fuel flow, the tailpipe temperature. When reduction or increase of turbine temperature is called for, the thermocouple signal tends to cause the tailpipe temperature motor I31 to rotate in a corresponding direction for temperature correction at potentiometer 51 until the desired temperature is reached when the motor is deenergized.

Referring specifically to Fig. 9, the tailpipe thermocouples I35 are terminated in a junction box I38 that is located as close as possible to the thermocouples but at a location where the ambient temperature is not excessively high. Since the thermocouple voltage signal is proportional to the temperature difference between the tailpipe and cold junction, it is necessary to compensate for the variation in generated voltage due to variation in ambient temperature. To this end a temperature responsive resistor I 39 is located in the junction box and is connected in the circuit so that the voltage variations across it substantially balance out the ambient temperature voltage component in the thermocouple output voltage.

The tailpipe temperature is compared with a standard or reference temperature in the following manner: a D. C. voltage corresponding to maximum permissible temperature, 1. e. the pre-set temperature value, is developed across a resistance I40. This voltage may be obtained from an A. C. sourceand is rectified and maintained constant by a rectifier and voltage regulating device generally indicated at I4I. For compensating variation in the voltage regulator output due to changing tubes, an adjustable potentiometer I42 is connected in the regulator output so as to maintain at terminal I43 a constant voltage of, say +104 volts D. C. where the nominal voltage supplied from regulator MI is slightly higher, say 105-volts D. C. The constant voltage junction I43 is connected to a plurality of voltage dividing circuits arranged in parallel, one circuit comprising a voltage divider having a high resistance I44 connected in series with the comparatively low cold-junction compensation resistance I39 which is in turn connected to ground so that the voltage drop across resistance I39 is but a small fraction of a volt.

Conductor 144a may thus be considered as a constant current source. As above indicated, the resistance I39 varies with ambient temperature, the resistor having a positive temperature coefiicient of resistance. Under conditions where no current flows through the thermocouple circuit, the voltage across resistor I39 is proportional to its resistance, thus providing the desired cold junction compensation. A second parallel circuit connected between junction I43 and ground includes series-connected resistances I45 and I40 proportioned so that a very small voltage is developed across the reference voltage resistance I4fl. Since the voltage at junction I43 remains constant and since the resistances I45 and I4!) are fixed, the reference voltage across resistance I46] remains fixed at the value representing the reference tailpipe or burner temperature, e. g. 1250 F.

The thermocouples I35, compensating resistance I 39 and reference resistance I40 are all connected in series with a pair of signal coils I46 and I4? of a pair of saturable reactors I48 and I49 respectivey of the sensing circuit I36, the various potentials being so related that for the pre-set temperature condition the compensated thermocouple voltage exactly balances the reference voltage so that no current fiows and the signal coils I45 and I41 are deenergized. Variation of thermocouple voltage so as to create an unbalance, either positive or negative with respect to the reference voltage across resistance I40 results in corresponding energization of the aforesaid signal coils. Each saturable reactor includes in addition to a D. C. signal coil, an A. C. coil and a flux biasing coil that functions as a flux reference. The biasing coil I50 of reactor I48 and the biasing coil I5I of reactor I49 are connected in series in a circuit that is energized from the constant voltage junction I43. The circuit includes a relatively high resistance I52 and is grounded so that the D. C. energization of the biasing or flux reference coils is constant at all times.

The signal coils I46 and I4! are connected in opposition to each other and are related to the respective biasing coils so that for a given D. C. signal; current the flux of one signal coil adds to the bias flux of its associated biasing coil whereas the flux of the other signal coil opposes or subtracts from the flux of the respective biasing coil. The reactors are designed so that normally when the signal coils are deepergized, i. e. when the pre-set temperature obtains, each reactor core is partially and equally saturated. Accordingly energization of the signal coils produces varying degrees of saturation in i the reactors and hence, since the impedance of a reactor coil varies with the degree of saturation of the core, the impedances of the A. C. reactor coils I53 and I54 vary accordingly.

The reactor coils I53 and I54 are connected in an alternating current bridge circuit I51 that is energized from an A. C. source E50 and connected to a detector I55 for obtaining a phased temperature signal. The bridge includes the two seriesconnected reactor coils I53 and I54 as legs on one side, and the series-connected resistances I 59a, I59!) and I590 on the opposite side. The junction terminals I56 and I560; vbetween the inductance and resistance sides of the bridge are connected to the source Em and ground respectively, the junction I56 being connected to the A. C. source through a resistance-condenser circuit I58 for insuring application of correct voltage and phase to the bridge. The opposite junction terminals pose of which is hereinafter explained. power lever contacts are closed only at the 100% power setting as indicated for fincreasetempera- I60 :andIfiIla are connected to an amplifying .detector I55, the terminals of which are shunted by a condenser I55a. This condenser functions to tune to approximate resonance the circuit including coils I53 and I54 for the purpose of increasing sensitivity. Junction IE is a mid-tap between the coils I53 and I54, and junction IBM- is an. adjustable resistancetap on the resistance I591) for initially balancing. the bridge so that no voltage appears across the junction terminals I60 and I600. when the signal coils are deenergized.

The detected temperature variation signal voltage from the amplifying detector I55 is used to control the tailpipe temperature motor [.31 in conventional manner. A pair of thermionic tubes I61 and H52 have their grid circuits connected in parallel to the detector as indicated, the output ofthe detector being connected to a common grid junction I63. The respective circuits of plates I64 and IE are arranged to be energized from the A. C. source E80 through transformer I-BB, the secondary winding I61 of which has a center tap connected by a conductor I63 to the control winding I69 of the two-phase motor I31.

A condenser'IIIl is connected across the motor winding I68 for conventional two-phase operation of themotorandl theireference voltage winding 'I'Il is connected through a condenser I72 and conductor M3. to the. reference A. C. source Eco.

For keeping this part of the circuit always in readiness for a decreased temperature correction, the plate I65 of tube IE2 is directly connected to the transformer secondary winding Hi1,

whereas the plate I64 of tube IEI is connected to the A. C. source in such. a way that the tailpipe temperature motor can call for increased temperatureonly under normal control and when the power lever is at the 100% power setting. That is, an increase in tailpipe temperature is essential. only when the realization of maximum power is desired, whereas the control for decreasing temperature shouldv be in readiness at all times and at all settings of the power lever for preventing turbine damage. Accordingly, power is decreased, if because of turbine malfunctioning maximum permissible temperature is exceeded at a reduced setting of thepower lever. For the above described purposes of control, the plate circuit-of tube IE2 is uninterrupted whereas the'plate circuit of tube I-8I includesin series the contacts 6% and I14 of the discriminator relay DR which are connected by conductor I1 5 to= thezpower lever contacts I18 and I'll. Also connected in series with contacts 68b and. H l are the normally closed contacts 2360 that are opened by actuation of. relay coil 236, the pur- The ture control and the relay contacts 58b and IM are closed during normal operation, i. e. when the control is connected to the computer output.

Assuming now that the thermocouples indicate excess temperature so that the thermocouple voltage' exceeds the reference voltage across resistance M0, the Voltage across the 1 series-connected signal coils I46 and Id! of the saturable reactor circuit has a. definite polarity, being. for example negativeat the upper terminal of coil [4.6 and positive. at the lower terminal of coil I41 for the excess temperature condition assumed. In case the thermocouple voltage is below the reference voltage indicating. that the tailpipe temperature can be increased, the polarity of the signal coil voltage is reversedas indicated. Depending upon its polarity the flux of a given signal coil adds to or opposes the bias flux of the associated bias coil so as to vary the impedance of the associated A. C. coil of the respective reactor. Concurrently, the othersignal coil is acting in opposite fashion on the bias flux of its associated bias coil so that the impedance of one A. C. coil is increased and that of the other is decreased. This changes the balance of the A. C. bridge I51 for producing the A. C. temlngiature signal voltage across junctions I60 and a. For the excess temperature condition above assumed, the amplified A. C. signal from detector I55 is in phase with the voltage at plate I65 of tube I 62 thereby causing the tube I62 to conduct and energize the motor winding IE9. The motor accordingly operates in a direction to adjust the temperature correction potentiometer 51, Fig. 4, so as to decrease the computed torque voltage and this voltage in turn controls the pitch changing mechanism for reducing the turbine torque and hence the tailpipe temperature. This reduced temperature correction is thus available at all times.

In. case the power lever is set for power under normal conditions and the thermocouple voltage is below the reference voltage indicating that the tailpipe temperature can be increased to the reference value for realizing maximum-permissibl-e torque, the bridge I51 of the saturable reactor circuit is unbalanced in the opposite direction and the A. C. signal voltage is displaced in phase with respect to that of the previously described decrease temperature signal. The signal voltage at grid junction I63 is now in phase with the voltage on plate I54 so that the tube IBI becomes conducting to energize motor winding I69. Since this energizing current is dephased 180 with respect to the previous control current the motor I3! is operated in the. opposite direction to adjust the temperature correction potentiometer so as to indicate increased computed torque. Therefore the turbine torque output is increased through. the servomotor BI and associated circuits, Figs. 4, 5 and 8, in the manner previously described. For obvious reasons, the tailpipe temperature motor is precluded by relay DR from making an increase temperature correction when an over-ride condition obtains, as for example when the ambient temperature and flight altitude are both low since the available torque is thus indicated to be already too high.

Because of the thermal lag in the thermocouples it is necessary to apply the correction comparatively slowly to avoid unstabilicing the.

control. Accordingly the gearing of tailpipe temperature motor i3! is such that the adjustment of the temperature correction potentiometer is at a rate producing a maximum rate of voltage'change corresponding for example to approximately 5%' of maximum torque per second. This maximum rate is attained when the temperature error is, say 100 F. or more.

Governor control specifically to Fig. 5, the governor assembly [80 comprises a main housing for the control valve spindle I82 and the fluid actuated servo system that controls the fuel valve. The flyball assembly for controlling the valve spindle is mounted Within an auxiliary housing 183 mounted on the main housing l8! and in communication. therewith through an opening 184 in alignment with the valve spindle. The valve spindle extends into the fiyball housing and is connected at its upper end to a disc 185 that is engaged at its lower surface by the toe arms B86 of the flyballs I81. The valve spindle 182 is also connected to a dished member I88 that is engaged by the lower end of the speeder spring I09, the upper end of which engages an adjustable abutment I90. Adjustment of the compression of the speeder spring for regulating the governor is effected by means of a governor motor I0! that is connected to the abutment I90 through gear reduction means including a pinion I92 and rack 193. The fly balls I81 are pivotally connected at 100 to a bracket structure I95 that constitutes an extension of a rotatable sleeve I96 suitably mounted in the housing 18! and connected by means including a gear i9! to the turbine shaft. The sleeve I90, which is apertured as illustrated and concentrically disposed on the control valve spindle S82, therefore is rotated according to turbine speed so that the fiyballs l8! which rotate with the sleeve are positioned, in case the turbine speed is increased to move the valve spindle upwardly against the tension of the speeder spring. When the turbine speed decreases the fiyballs tend to collapse and the valve spindle is depressed by the speeder spring until the spring force and flyball centrifugal force balance each other. The iiyball toe arms- I86 rock on the lower surface of the disc I85 so that the effects of friction are minimized. It will be understood that the governor apparatus is specifically described only by Way of example and that other governor mechanisms having suitable characteristics may be used in the practice of the present invention.

The governor is hydraulically controlled in response to flyball operation of the valve spindle for operating the fuel control element 103 in the following manner: the space between the two bottom lands I99 and 200 of the valve spindle is arranged to be in communication with a passage 201 connected to a source of servo oil under pressure for controlling application of servo oil pres sure to the fuel servo piston 202 in conventional manner. In response to a turbine underspeed for example the speeder spring 09 pushes the U valve spindle downward a certain amount so as to open the passage 203 leading to the servo cylin-, der 204. This passage also communicates with the buifer cylinder 205 and includes an adjustable needle valve 206. The resulting surge of oil deflects the buffer piston 2051) which tends to be centered as illustrated by springs 207 and produces a pressure differential proportional to the buffer spring deflection on the compensating piston 208 that is connected to the control valve spindle. This piston is in communication with buffer cylinder 205 by passage 205a and with passage 203 by passage 20301. It is apparent that the pressure at opposite sides of the compensating piston 208 is equalized in the steady-state condition by reason of leakage of servo fluid through needle valve 206.

For the increase-speed signal in question the differential force above described urges the valve spindle in a direction tending to reclose the metering valve 200a. The action is such that a quantity of oil is rapidly admitted to servo cylinder 204, this quantity being proportional to the off-speed. The needle valve 200 located in the passage 203 between the buffer piston and servopiston is adjusted so that the flow of oil through the needle valve into the servo cylinder is maintained as long as the R. P. M. error persists. As oil is admitted into the servo cylinder, the servo piston 202 compresses a restoring spring 209 so that the servo cylinder oil pressure is proportional to the servo piston oil displacement. The rate of oil flow at the needle valve is approximately proportional to the buffer spring deflection and consequently to engine off-speed. Thus, the change of position of the servo piston due to this component of oil flow is proportional to the time integral of the off-speed. Therefore since the fuel flow to the turbine is to correspond with the position of the spring restored servo piston 202, it will be noted that change in fuel flow is proportional to off-speed plus the time integral of the offspeed. This fulfills the conditions required for a droop stabilized governor, i. e. wherein the R. P. M. is allowed to droop slightly for a brief period following sudden torque changes.

The displacement of the servo piston constitutes the output signal of the governor and is applied in suitable manner to operate a control valve Va in the fuel line 4 as indicated in Fig. 10, or for example by controlling a conventional fuel pump displacement valve (not'shown) ordinarily supplied with the turbine. For this latter case it may be the pressure inside cylinder 204 rather than the displacement of piston 202 which effects the control. The action of the fuel pump displacement control valve is such that the stroke of the fuel pump adjusts itself until fuel pressure is proportional to the governor output pressure signal, the resulting fuel flow being essentially proportional only to this pressure and independent of combustion chamber pressure.

The speed setting of the governor is controlled as above indicated by an electric servomotor, Fig. 10, that operates through gearing I92 and I93 to adjust the speeder spring I89. The gear reduction between the electric motor and speeder spring is in the present instance, designed so that about 20 seconds is required for the servomotor to displace the speeder spring through its full stroke from a setting of, for example, 8000 R. P. M. to 13,000 R. P. M. of the turbine. The reason for this slow action is as previously mentioned that large amounts of kinetic energy must be added to the rotating part of the turbine to increase its speed. Consequently the acceleration of the turbine at a rate of 250 R. P. M. per second requires a substantial increase of turbine temperature above its steady state value. It is therefore necessary to restrict the rate at which the governor speed setting can be changed so that excessive amounts of fuel are not applied by the governor due to a large speed error being suddenly introduced, such as when the condition lever is moved from the 8000 to the 13,000 R. P. M. position, i. e. from ground idle to take-off and landing. However, as above indicated, maximum rate of acceleration depends on the turbine temperature overshoot that can be tolerated and the above data is merely explanatory.

Since in the present case the minimum setting of the governor is for 8000 R. P. M. at start and ground idle, the governor will at the low R. P. M. existing during turbine starting call however be carefully controlled during the starting operation, an auxiliary control in addition to the governor is required. A manually controlled fuel valve-Vin, Fig. 3, may be used during this operation so as to be manually operated by a direct linkage connection with the fuel lever l1 in the cockpit and thus directly to control the fuel flow. This valve may also be used for stopping the turbine by shutting off the fuel as previously described in explaining control action during feathering. Obviously the governor cannot be used for this operation since if it permitted the fuel to be completely cutoff the burners might be accidentally extinguished during a transient overspeed condition.

The control of the electric servomotcr for adjusting the governor setting according to condition and power lever operation is illustrated by Fig. 10'. The motor I!!! is of the two-phase type and is connected in an automatic balancing circuit in such manner thatthe motor operates through a range and in a direction corresponding to the magnitude and sense of an unbalance voltage produced according to certain settings of the condition and power levers. Specifically, the balancing circuit 210 is energized from an alternating current source Etc through a transformer 2, a secondary winding 212 of which is connected to conductors 213 and 2M for en'- ergizing three resistance circuits 2&5, 2l-6 and 2-H arranged in parallel. The resistance circuit 215 is provided with voltage taps 2H! and 2H! that are connected to certain contacts 2280 and. 228a of the condition lever Ill representing two different turbine speeds for controlling the governor motor circuit in a manner presently described. The resistance circuit 2I6 is also arranged to be alternatively connected by the power and condition levers to the motor control circuit. This connection includes a. slider contact 22!!- positioned by the power lever 9, conductor 220 and the contacts 228 and 228d of the condition lever which are connected to the motor control in a manner presently described. The resistance circuit 211 constitutes a balancing potentiometer having a slider contact 22! that is positioned by the motor l9l through a suitable mechanical connection indicated at 222.

The motor energizing circuit comprises a pair of windings 223 and 224, the winding 223 being energized from the reference SOuI'CBEac and the control winding 224 being energized from a pair of thermionic tubes 225 and 226 that are in turn controlled by the unbalance voltage output of amplifier 227. This output depends on the voltage selected by the condition lever, or by the condition lever in combination with the power lever for the special case of normal operation wherein either full R. P. M. or predetermined lower R. P. M.s are alternatively available.

The condition lever is connected to the slider contact 228 for engaging the associated fixed contacts corresponding to the condition lever positions as indicated, and the slider contact is connected by a conductor 229 to the input of the amplifier 221, the output of which is connected by conductor 230 to a common junction 23! of the parallel connected control grid circuits of the tubes 225 and 226. The plate circuits 1232 and 233 of the respective tubes are connected to a secondary winding 234 of the transformer 2-H and the winding 234 is connected at a center tap as indicated to the motor control winding 224' by conductor 235 to constitute the common return andilo'ad circuit of 26 the tubes.

a relay coil 236 shunted "by a condenser 238a. The relay coil controls a pair of contacts 2361) connected in the input conductor 87 of amplifierilt, Fig 4, for the purpose of disabling during an increase of R. P. M. the power servo-- motor 8!, and also causes the temperature-- increase circuits of the temperature correction servomotor to be disabled by opening the circuit through contacts 2360 (Fig. 9). Thus for an increase R. P. M. condition'the accompanying, power increase indicated in Fig. 11 is not 'made:

until the R. P. M. change has been essentially completed, thereby permitting the transient temsentially the same as that described in Fig. 9 is unnecessary, it being sufiicient to state that tube 225 is made conducting for energizing the motor control winding 2% in one direction to increase the'R. P. M. when the voltage impressed on the grid is in phase with reference valtage Eat, and

alternatively the tube 226 is made conducting when the grid voltage is dephased by so that the motor control winding 22 i is energized in the opposite direction for reversing the rotation of the motor and decreasing R. P. M.

The phase relationship of the control voltage input to amplifier 227 is determined according to the position of the slider contact 22-] of'potentiometer 25? relative to the voltage position selected at the branch resistance circuit 215', 2 [For 213 as the case may be. Since the amplifier'ZZ-l is arranged to be included in the balancing circuit between the branch circuits and the slider 221;

the amplifier input will be zero and the motor lei will be deenergized when for example thecondition setting is at take-off and landing" and the voltage from terminal conductor 2P3 'to the tap 2J9 of branch circuit H5 is equal to the voltage from conductor 2 A3 to the potentiometer slider 221, thus balancing the circuit for the governor setting at maximum R. P. When however, these voltages are unequal, as whenthe potentiometer voltage is less than the other voltage, the phase of the unbalance input voltage to amplifier 221 has a definite relation to the phase ofthc reference voltage Eac, and when the potentiometer voltage is greater than the other voltage the amplifier input voltage is dephased 180" with respect to the former input.

From the above description of th'emotor control and balancing circuits, it will "be seen that with the motor l9! polarized so as to move the potentiometer slider 22! in a directionto decrease" the voltage difference across the circuit including the amplifier 22 1, the servo system tends to maintain an automatic balance and to operate the governormotor l9! according to the magnitudeand' sense of the voltage unbalance established by operation of the condition and power levers.

Pilots control quadrant The control lever system used in the present invention follows the principle of the so-called cording to a predetermined schedule thatvar-ies with different-flight conditions.

A second con The plate circuit'232 of the tube 225 which is the increase R. 'P. M. circuit includes In the present case, turbine power and 

